Priority is claimed to Application No. 00-15035, filed in the Republic of Korea on Mar. 24, 2000, herein incorporated by reference.
1. Field of the Invention
The present invention relates to encryption by sharing a secrete key, and more particularly, to a key agreement method in a secure communication system using a multiple access method.
2. Description of the Related Art
As communication services have significantly increased and information-oriented society has come, system security and reliability have become issues of great importance. In particular, as services such as an electronic commercial or financial transaction and a network information service increase, the need to protect personal information increases. Accordingly, cryptosystem has become more important than ever.
In cryptosystems, a key is used as an input of encryption or decryption function for scrambling plain text. In the cipher system where plain text is encoded and decoded by encryption and decryption algorithms, the ciphered text and the encryption algorithm are available along the public communication channels for not only legitimate users but also illegitimate users. Therefore, it is important to distribute, store, manage securely keys that are exclusive for legitimate users, and security of a system mainly depends on the security of the keys.
Most conventional cryptosystems use algorithms based on computational complexity and are vulnerable to attacks with a powerful computer as well as physical attacks. Moreover, one of drawbacks of the conventional cryptosystem is that it can always be monitored without the legitimate users being aware of any eavesdropping taking place. When distribution or agreement of private keys are performed via private channels, no matter how robust the physical channel is, there are always chances of the channel being attacked by a malicious eavesdropper. The eavesdropper may be able to obtain the value of the key by tapping or cloning the transmitted key signals, produce the signals based on the measured value, and re-transmit the signals to the other legitimate user. Therefore, the legitimate users cannot be sure whether the communication line is attacked, or when eavesdropping occurs.
Conventional public-key cryptosystems are based on computational complexity. But the advent of a powerful computer working on new algorithms such as the quantum computer has been a threat to the conventional cryptosystems.
For example, methods using complex algorithms include the public key, knapsack method (U.S. Pat. No. 4,218,582) and RSA (Rivest, Shamir, and Adleman) system (U.S. Pat. No. 4,405,829), and these methods are based on mathematical complexity. The cipher systems not based on mathematical complexity include the key distributing system by quantum cryptology (U.S. Pat. No. 5,307,410 and U.S. Pat. No. 5,515,438). However, since light used in the quantum cryptosystem should be in the single photon state, in the coherent state with very low power, or in the state which is difficult to generate, it has many obstacles in utilizing the quantum cryptosystem in practice in the very near future. Furthermore, since signal amplification is impossible in quantum cryptosystem, it is not suitable for long-haul communication.